/**
 * @file
 * DNS - host name to IP address resolver.
 *
 */

/**

 * This file implements a DNS host name to IP address resolver.

 * Port to lwIP from uIP
 * by Jim Pettinato April 2007

 * uIP version Copyright (c) 2002-2003, Adam Dunkels.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * DNS.C
 *
 * The lwIP DNS resolver functions are used to lookup a host name and
 * map it to a numerical IP address. It maintains a list of resolved
 * hostnames that can be queried with the dns_lookup() function.
 * New hostnames can be resolved using the dns_query() function.
 *
 * The lwIP version of the resolver also adds a non-blocking version of
 * gethostbyname() that will work with a raw API application. This function
 * checks for an IP address string first and converts it if it is valid.
 * gethostbyname() then does a dns_lookup() to see if the name is
 * already in the table. If so, the IP is returned. If not, a query is
 * issued and the function returns with a ERR_INPROGRESS status. The app
 * using the dns client must then go into a waiting state.
 *
 * Once a hostname has been resolved (or found to be non-existent),
 * the resolver code calls a specified callback function (which
 * must be implemented by the module that uses the resolver).
 */

/*-----------------------------------------------------------------------------
* RFC 1035 - Domain names - implementation and specification
* RFC 2181 - Clarifications to the DNS Specification
*----------------------------------------------------------------------------*/

/** @todo: define good default values (rfc compliance) */
/** @todo: improve answer parsing, more checkings... */
/** @todo: check RFC1035 - 7.3. Processing responses */

/*-----------------------------------------------------------------------------
* Includes
*----------------------------------------------------------------------------*/

#include "lwip/opt.h"

#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */

#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/dns.h"

#include <string.h>

/** DNS server IP address */
#ifndef DNS_SERVER_ADDRESS
#define DNS_SERVER_ADDRESS        inet_addr("208.67.222.222") /* resolver1.opendns.com */
#endif

/** DNS server port address */
#ifndef DNS_SERVER_PORT
#define DNS_SERVER_PORT           53
#endif

/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
#define DNS_MAX_RETRIES           4
#endif

/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
#define DNS_MAX_TTL               604800
#endif

/* DNS protocol flags */
#define DNS_FLAG1_RESPONSE        0x80
#define DNS_FLAG1_OPCODE_STATUS   0x10
#define DNS_FLAG1_OPCODE_INVERSE  0x08
#define DNS_FLAG1_OPCODE_STANDARD 0x00
#define DNS_FLAG1_AUTHORATIVE     0x04
#define DNS_FLAG1_TRUNC           0x02
#define DNS_FLAG1_RD              0x01
#define DNS_FLAG2_RA              0x80
#define DNS_FLAG2_ERR_MASK        0x0f
#define DNS_FLAG2_ERR_NONE        0x00
#define DNS_FLAG2_ERR_NAME        0x03

/* DNS protocol states */
#define DNS_STATE_UNUSED          0
#define DNS_STATE_NEW             1
#define DNS_STATE_ASKING          2
#define DNS_STATE_DONE            3

#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS message header */
struct dns_hdr
{
    PACK_STRUCT_FIELD(u16_t id);
    PACK_STRUCT_FIELD(u8_t flags1);
    PACK_STRUCT_FIELD(u8_t flags2);
    PACK_STRUCT_FIELD(u16_t numquestions);
    PACK_STRUCT_FIELD(u16_t numanswers);
    PACK_STRUCT_FIELD(u16_t numauthrr);
    PACK_STRUCT_FIELD(u16_t numextrarr);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_HDR 12

#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS query message structure */
struct dns_query
{
    /* DNS query record starts with either a domain name or a pointer
       to a name already present somewhere in the packet. */
    PACK_STRUCT_FIELD(u16_t type);
    PACK_STRUCT_FIELD(u16_t class);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_QUERY 4

#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS answer message structure */
struct dns_answer
{
    /* DNS answer record starts with either a domain name or a pointer
       to a name already present somewhere in the packet. */
    PACK_STRUCT_FIELD(u16_t type);
    PACK_STRUCT_FIELD(u16_t class);
    PACK_STRUCT_FIELD(u32_t ttl);
    PACK_STRUCT_FIELD(u16_t len);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#  include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_ANSWER 10

/** DNS table entry */
struct dns_table_entry
{
    u8_t state;
    u8_t numdns;
    u8_t tmr;
    u8_t retries;
    u8_t seqno;
    u8_t err;
    u32_t ttl;
    char name[DNS_MAX_NAME_LENGTH];
    struct ip_addr ipaddr;
    /* pointer to callback on DNS query done */
    dns_found_callback found;
    void *arg;
};

#if DNS_LOCAL_HOSTLIST
/** struct used for local host-list */
struct local_hostlist_entry
{
    /** static hostname */
    const char *name;
    /** static host address in network byteorder */
    u32_t addr;
    struct local_hostlist_entry *next;
};

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Local host-list. For hostnames in this list, no
 *  external name resolution is performed */
static struct local_hostlist_entry *local_hostlist_dynamic;
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

/** Defining this allows the local_hostlist_static to be placed in a different
 * linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
#define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
/** Defining this allows the local_hostlist_static to be placed in a different
 * linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
#define DNS_LOCAL_HOSTLIST_STORAGE_POST
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[]
DNS_LOCAL_HOSTLIST_STORAGE_POST = DNS_LOCAL_HOSTLIST_INIT;

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

static void dns_init_local();
#endif /* DNS_LOCAL_HOSTLIST */


/* forward declarations */
static void dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port);
static void dns_check_entries(void);

/*-----------------------------------------------------------------------------
* Globales
*----------------------------------------------------------------------------*/

/* DNS variables */
static struct udp_pcb        *dns_pcb;
static u8_t dns_seqno;
static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
static struct ip_addr dns_servers[DNS_MAX_SERVERS];

#if (DNS_USES_STATIC_BUF == 1)
static u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 1) */

/**
 * Initialize the resolver: set up the UDP pcb and configure the default server
 * (DNS_SERVER_ADDRESS).
 */
void
dns_init()
{
    struct ip_addr dnsserver;

    /* initialize default DNS server address */
    dnsserver.addr = DNS_SERVER_ADDRESS;

    LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));

    /* if dns client not yet initialized... */
    if (dns_pcb == NULL)
    {
        dns_pcb = udp_new();

        if (dns_pcb != NULL)
        {
            /* initialize DNS table not needed (initialized to zero since it is a
             * global variable) */
            LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
                        DNS_STATE_UNUSED == 0);

            /* initialize DNS client */
            udp_bind(dns_pcb, IP_ADDR_ANY, 0);
            udp_recv(dns_pcb, dns_recv, NULL);

            /* initialize default DNS primary server */
            dns_setserver(0, &dnsserver);
        }
    }
#if DNS_LOCAL_HOSTLIST
    dns_init_local();
#endif
}

/**
 * Initialize one of the DNS servers.
 *
 * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
 * @param dnsserver IP address of the DNS server to set
 */
void
dns_setserver(u8_t numdns, struct ip_addr *dnsserver)
{
    if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
        (dnsserver != NULL) && (dnsserver->addr != 0 ))
    {
        dns_servers[numdns] = (*dnsserver);
    }
}

/**
 * Obtain one of the currently configured DNS server.
 *
 * @param numdns the index of the DNS server
 * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
 *         server has not been configured.
 */
struct ip_addr
dns_getserver(u8_t numdns)
{
    if (numdns < DNS_MAX_SERVERS)
    {
        return dns_servers[numdns];
    }
    else
    {
        return *IP_ADDR_ANY;
    }
}

/**
 * The DNS resolver client timer - handle retries and timeouts and should
 * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
 */
void
dns_tmr(void)
{
    if (dns_pcb != NULL)
    {
        LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
        dns_check_entries();
    }
}

#if DNS_LOCAL_HOSTLIST
static void
dns_init_local()
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT)
    int i;
    struct local_hostlist_entry *entry;
    /* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
    struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;
    for (i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++)
    {
        entry = mem_malloc(sizeof(struct local_hostlist_entry));
        LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);
        if (entry != NULL)
        {
            struct local_hostlist_entry *init_entry = &local_hostlist_init[i];
            entry->name = init_entry->name;
            entry->addr = init_entry->addr;
            entry->next = local_hostlist_dynamic;
            local_hostlist_dynamic = entry;
        }
    }
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
}

/**
 * Scans the local host-list for a hostname.
 *
 * @param hostname Hostname to look for in the local host-list
 * @return The first IP address for the hostname in the local host-list or
 *         INADDR_NONE if not found.
 */
static u32_t
dns_lookup_local(const char *hostname)
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
    struct local_hostlist_entry *entry = local_hostlist_dynamic;
    while (entry != NULL)
    {
        if (strcmp(entry->name, hostname) == 0)
        {
            return entry->addr;
        }
        entry = entry->next;
    }
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
    int i;
    for (i = 0; i < sizeof(local_hostlist_static) / sizeof(struct local_hostlist_entry); i++)
    {
        if (strcmp(local_hostlist_static[i].name, hostname) == 0)
        {
            return local_hostlist_static[i].addr;
        }
    }
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
    return INADDR_NONE;
}

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Remove all entries from the local host-list for a specific hostname
 * and/or IP addess
 *
 * @param hostname hostname for which entries shall be removed from the local
 *                 host-list
 * @param addr address for which entries shall be removed from the local host-list
 * @return the number of removed entries
 */
int
dns_local_removehost(const char *hostname, const struct ip_addr *addr)
{
    int removed = 0;
    struct local_hostlist_entry *entry = local_hostlist_dynamic;
    struct local_hostlist_entry *last_entry = NULL;

    while (entry != NULL)
    {
        if (((hostname == NULL) || !strcmp(entry->name, hostname)) &&
            ((addr == NULL) || (entry->addr == addr->addr)))
        {
            struct local_hostlist_entry *free_entry;
            if (last_entry != NULL)
            {
                last_entry->next = entry->next;
            }
            else
            {
                local_hostlist_dynamic = entry->next;
            }
            free_entry = entry;
            entry = entry->next;
            mem_free(free_entry);
            removed++;
        }
        else
        {
            last_entry = entry;
            entry = entry->next;
        }
    }
    return removed;
}

/**
 * Add a hostname/IP address pair to the local host-list.
 * Duplicates are not checked.
 *
 * @param hostname hostname of the new entry
 * @param addr IP address of the new entry
 * @return ERR_OK if succeeded or ERR_MEM on memory error
 */
err_t
dns_local_addhost(const char *hostname, const struct ip_addr *addr)
{
    struct local_hostlist_entry *entry;

    entry = mem_malloc(sizeof(struct local_hostlist_entry));
    if (entry == NULL)
    {
        return ERR_MEM;
    }
    entry->name = hostname;
    entry->addr = addr->addr;
    entry->next = local_hostlist_dynamic;
    local_hostlist_dynamic = entry;
    return ERR_OK;
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
#endif /* DNS_LOCAL_HOSTLIST */

/**
 * Look up a hostname in the array of known hostnames.
 *
 * @note This function only looks in the internal array of known
 * hostnames, it does not send out a query for the hostname if none
 * was found. The function dns_enqueue() can be used to send a query
 * for a hostname.
 *
 * @param name the hostname to look up
 * @return the hostname's IP address, as u32_t (instead of struct ip_addr to
 *         better check for failure: != INADDR_NONE) or INADDR_NONE if the hostname
 *         was not found in the cached dns_table.
 */
static u32_t
dns_lookup(const char *name)
{
    u8_t i;

#if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
    u32_t addr;
#endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
#if DNS_LOCAL_HOSTLIST
    if ((addr = dns_lookup_local(name)) != INADDR_NONE)
    {
        return addr;
    }
#endif /* DNS_LOCAL_HOSTLIST */
#ifdef DNS_LOOKUP_LOCAL_EXTERN
    if ((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != INADDR_NONE)
    {
        return addr;
    }
#endif /* DNS_LOOKUP_LOCAL_EXTERN */

    /* Walk through name list, return entry if found. If not, return NULL. */
    for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
        if ((dns_table[i].state == DNS_STATE_DONE) &&
            (strcmp(name, dns_table[i].name) == 0))
        {
            LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
            ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
            LWIP_DEBUGF(DNS_DEBUG, ("\n"));
            return dns_table[i].ipaddr.addr;
        }
    }

    return INADDR_NONE;
}

#if DNS_DOES_NAME_CHECK
/**
 * Compare the "dotted" name "query" with the encoded name "response"
 * to make sure an answer from the DNS server matches the current dns_table
 * entry (otherwise, answers might arrive late for hostname not on the list
 * any more).
 *
 * @param query hostname (not encoded) from the dns_table
 * @param response encoded hostname in the DNS response
 * @return 0: names equal; 1: names differ
 */
static u8_t
dns_compare_name(unsigned char *query, unsigned char *response)
{
    unsigned char n;

    do
    {
        n = *response++;
        /** @see RFC 1035 - 4.1.4. Message compression */
        if ((n & 0xc0) == 0xc0)
        {
            /* Compressed name */
            break;
        }
        else
        {
            /* Not compressed name */
            while (n > 0)
            {
                if ((*query) != (*response))
                {
                    return 1;
                }
                ++response;
                ++query;
                --n;
            }
            ;
            ++query;
        }
    }
    while (*response != 0);

    return 0;
}
#endif /* DNS_DOES_NAME_CHECK */

/**
 * Walk through a compact encoded DNS name and return the end of the name.
 *
 * @param query encoded DNS name in the DNS server response
 * @return end of the name
 */
static unsigned char *
dns_parse_name(unsigned char *query)
{
    unsigned char n;

    do
    {
        n = *query++;
        /** @see RFC 1035 - 4.1.4. Message compression */
        if ((n & 0xc0) == 0xc0)
        {
            /* Compressed name */
            break;
        }
        else
        {
            /* Not compressed name */
            while (n > 0)
            {
                ++query;
                --n;
            }
            ;
        }
    }
    while (*query != 0);

    return query + 1;
}

/**
 * Send a DNS query packet.
 *
 * @param numdns index of the DNS server in the dns_servers table
 * @param name hostname to query
 * @param id index of the hostname in dns_table, used as transaction ID in the
 *        DNS query packet
 * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
 */
static err_t
dns_send(u8_t numdns, const char* name, u8_t id)
{
    err_t err;
    struct dns_hdr *hdr;
    struct dns_query qry;
    struct pbuf *p;
    char *query, *nptr;
    const char *pHostname;
    u8_t n;

    LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U 16_F "] \"%s\": request\n",
                            (u16_t)(numdns), name));
    LWIP_ASSERT("dns server out of array", numdns < DNS_MAX_SERVERS);
    LWIP_ASSERT("dns server has no IP address set", dns_servers[numdns].addr != 0);

    /* if here, we have either a new query or a retry on a previous query to process */
    p = pbuf_alloc(PBUF_TRANSPORT, SIZEOF_DNS_HDR + DNS_MAX_NAME_LENGTH +
                   SIZEOF_DNS_QUERY, PBUF_RAM);
    if (p != NULL)
    {
        LWIP_ASSERT("pbuf must be in one piece", p->next == NULL);
        /* fill dns header */
        hdr = (struct dns_hdr*)p->payload;
        memset(hdr, 0, SIZEOF_DNS_HDR);
        hdr->id = htons(id);
        hdr->flags1 = DNS_FLAG1_RD;
        hdr->numquestions = htons(1);
        query = (char*)hdr + SIZEOF_DNS_HDR;
        pHostname = name;
        --pHostname;

        /* convert hostname into suitable query format. */
        do
        {
            ++pHostname;
            nptr = query;
            ++query;
            for (n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname)
            {
                *query = *pHostname;
                ++query;
                ++n;
            }
            *nptr = n;
        }
        while (*pHostname != 0);
        *query++ = '\0';

        /* fill dns query */
        qry.type  = htons(DNS_RRTYPE_A);
        qry.class = htons(DNS_RRCLASS_IN);
        MEMCPY( query, &qry, SIZEOF_DNS_QUERY);

        /* resize pbuf to the exact dns query */
        pbuf_realloc(p, (query + SIZEOF_DNS_QUERY) - ((char*)(p->payload)));

        /* connect to the server for faster receiving */
        udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
        /* send dns packet */
        err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);

        /* free pbuf */
        pbuf_free(p);
    }
    else
    {
        err = ERR_MEM;
    }

    return err;
}

/**
 * dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void
dns_check_entry(u8_t i)
{
    struct dns_table_entry *pEntry = &dns_table[i];

    LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);

    switch (pEntry->state)
    {

    case DNS_STATE_NEW: {
        /* initialize new entry */
        pEntry->state   = DNS_STATE_ASKING;
        pEntry->numdns  = 0;
        pEntry->tmr     = 1;
        pEntry->retries = 0;

        /* send DNS packet for this entry */
        dns_send(pEntry->numdns, pEntry->name, i);
        break;
    }

    case DNS_STATE_ASKING: {
        if (--pEntry->tmr == 0)
        {
            if (++pEntry->retries == DNS_MAX_RETRIES)
            {
                if ((pEntry->numdns + 1 < DNS_MAX_SERVERS) && (dns_servers[pEntry->numdns + 1].addr != 0))
                {
                    /* change of server */
                    pEntry->numdns++;
                    pEntry->tmr     = 1;
                    pEntry->retries = 0;
                    break;
                }
                else
                {
                    LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", pEntry->name));
                    /* call specified callback function if provided */
                    if (pEntry->found)
                    {
                        (*pEntry->found)(pEntry->name, NULL, pEntry->arg);
                    }
                    /* flush this entry */
                    pEntry->state   = DNS_STATE_UNUSED;
                    pEntry->found   = NULL;
                    break;
                }
            }

            /* wait longer for the next retry */
            pEntry->tmr = pEntry->retries;

            /* send DNS packet for this entry */
            dns_send(pEntry->numdns, pEntry->name, i);
        }
        break;
    }

    case DNS_STATE_DONE: {
        /* if the time to live is nul */
        if (--pEntry->ttl == 0)
        {
            LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
            /* flush this entry */
            pEntry->state = DNS_STATE_UNUSED;
            pEntry->found = NULL;
        }
        break;
    }
    case DNS_STATE_UNUSED:
        /* nothing to do */
        break;
    default:
        LWIP_ASSERT("unknown dns_table entry state:", 0);
        break;
    }
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void
dns_check_entries(void)
{
    u8_t i;

    for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
        dns_check_entry(i);
    }
}

/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 *
 * @params see udp.h
 */
static void
dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port)
{
    u8_t i;
    char *pHostname;
    struct dns_hdr *hdr;
    struct dns_answer ans;
    struct dns_table_entry *pEntry;
    u8_t nquestions, nanswers;

#if (DNS_USES_STATIC_BUF == 0)
    u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 0) */
#if (DNS_USES_STATIC_BUF == 2)
    u8_t* dns_payload;
#endif /* (DNS_USES_STATIC_BUF == 2) */

    LWIP_UNUSED_ARG(arg);
    LWIP_UNUSED_ARG(pcb);
    LWIP_UNUSED_ARG(addr);
    LWIP_UNUSED_ARG(port);

    /* is the dns message too big ? */
    if (p->tot_len > DNS_MSG_SIZE)
    {
        LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
        /* free pbuf and return */
        goto memerr1;
    }

    /* is the dns message big enough ? */
    if (p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_DNS_ANSWER))
    {
        LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
        /* free pbuf and return */
        goto memerr1;
    }

#if (DNS_USES_STATIC_BUF == 2)
    dns_payload = mem_malloc(p->tot_len);
    if (dns_payload == NULL)
    {
        LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: mem_malloc error\n"));
        /* free pbuf and return */
        goto memerr1;
    }
#endif /* (DNS_USES_STATIC_BUF == 2) */

    /* copy dns payload inside static buffer for processing */
    if (pbuf_copy_partial(p, dns_payload, p->tot_len, 0) == p->tot_len)
    {
        /* The ID in the DNS header should be our entry into the name table. */
        hdr = (struct dns_hdr*)dns_payload;
        i = htons(hdr->id);
        if (i < DNS_TABLE_SIZE)
        {
            pEntry = &dns_table[i];
            if (pEntry->state == DNS_STATE_ASKING)
            {
                /* This entry is now completed. */
                pEntry->state = DNS_STATE_DONE;
                pEntry->err   = hdr->flags2 & DNS_FLAG2_ERR_MASK;

                /* We only care about the question(s) and the answers. The authrr
                   and the extrarr are simply discarded. */
                nquestions = htons(hdr->numquestions);
                nanswers   = htons(hdr->numanswers);

                /* Check for error. If so, call callback to inform. */
                if (((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0) || (pEntry->err != 0) || (nquestions != 1))
                {
                    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", pEntry->name));
                    /* call callback to indicate error, clean up memory and return */
                    goto responseerr;
                }

#if DNS_DOES_NAME_CHECK
                /* Check if the name in the "question" part match with the name in the entry. */
                if (dns_compare_name((unsigned char*)(pEntry->name), (unsigned char*)dns_payload + SIZEOF_DNS_HDR) != 0)
                {
                    LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
                    /* call callback to indicate error, clean up memory and return */
                    goto responseerr;
                }
#endif          /* DNS_DOES_NAME_CHECK */

                /* Skip the name in the "question" part */
                pHostname = (char*)dns_parse_name((unsigned char*)dns_payload + SIZEOF_DNS_HDR) + SIZEOF_DNS_QUERY;

                while (nanswers > 0)
                {
                    /* skip answer resource record's host name */
                    pHostname = (char*)dns_parse_name((unsigned char*)pHostname);

                    /* Check for IP address type and Internet class. Others are discarded. */
                    MEMCPY(&ans, pHostname, SIZEOF_DNS_ANSWER);
                    if ((ntohs(ans.type) == DNS_RRTYPE_A) && (ntohs(ans.class) == DNS_RRCLASS_IN) && (ntohs(ans.len) == sizeof(struct ip_addr)) )
                    {
                        /* read the answer resource record's TTL, and maximize it if needed */
                        pEntry->ttl = ntohl(ans.ttl);
                        if (pEntry->ttl > DNS_MAX_TTL)
                        {
                            pEntry->ttl = DNS_MAX_TTL;
                        }
                        /* read the IP address after answer resource record's header */
                        MEMCPY( &(pEntry->ipaddr), (pHostname + SIZEOF_DNS_ANSWER), sizeof(struct ip_addr));
                        LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", pEntry->name));
                        ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
                        LWIP_DEBUGF(DNS_DEBUG, ("\n"));
                        /* call specified callback function if provided */
                        if (pEntry->found)
                        {
                            (*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg);
                        }
                        /* deallocate memory and return */
                        goto memerr2;
                    }
                    else
                    {
                        pHostname = pHostname + SIZEOF_DNS_ANSWER + htons(ans.len);
                    }
                    --nanswers;
                }
                LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", pEntry->name));
                /* call callback to indicate error, clean up memory and return */
                goto responseerr;
            }
        }
    }

    /* deallocate memory and return */
    goto memerr2;

responseerr:
    /* ERROR: call specified callback function with NULL as name to indicate an error */
    if (pEntry->found)
    {
        (*pEntry->found)(pEntry->name, NULL, pEntry->arg);
    }
    /* flush this entry */
    pEntry->state = DNS_STATE_UNUSED;
    pEntry->found = NULL;

memerr2:
#if (DNS_USES_STATIC_BUF == 2)
    /* free dns buffer */
    mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */

memerr1:
    /* free pbuf */
    pbuf_free(p);
    return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param found a callback founction to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return @return a err_t return code.
 */
static err_t
dns_enqueue(const char *name, dns_found_callback found, void *callback_arg)
{
    u8_t i;
    u8_t lseq, lseqi;
    struct dns_table_entry *pEntry = NULL;

    /* search an unused entry, or the oldest one */
    lseq = lseqi = 0;
    for (i = 0; i < DNS_TABLE_SIZE; ++i)
    {
        pEntry = &dns_table[i];
        /* is it an unused entry ? */
        if (pEntry->state == DNS_STATE_UNUSED)
        {
            break;
        }

        /* check if this is the oldest completed entry */
        if (pEntry->state == DNS_STATE_DONE)
        {
            if ((dns_seqno - pEntry->seqno) > lseq)
            {
                lseq = dns_seqno - pEntry->seqno;
                lseqi = i;
            }
        }
    }

    /* if we don't have found an unused entry, use the oldest completed one */
    if (i == DNS_TABLE_SIZE)
    {
        if ((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE))
        {
            /* no entry can't be used now, table is full */
            LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
            return ERR_MEM;
        }
        else
        {
            /* use the oldest completed one */
            i = lseqi;
            pEntry = &dns_table[i];
        }
    }

    /* use this entry */
    LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %"U 16_F "\n", name, (u16_t)(i)));

    /* fill the entry */
    pEntry->state = DNS_STATE_NEW;
    pEntry->seqno = dns_seqno++;
    pEntry->found = found;
    pEntry->arg   = callback_arg;
    strcpy(pEntry->name, name);

    /* force to send query without waiting timer */
    dns_check_entry(i);

    /* dns query is enqueued */
    return ERR_INPROGRESS;
}

/**
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a struct ip_addr where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t
dns_gethostbyname(const char *hostname, struct ip_addr *addr, dns_found_callback found,
                  void *callback_arg)
{
    /* not initialized or no valid server yet, or invalid addr pointer
     * or invalid hostname or invalid hostname length */
    if ((dns_pcb == NULL) || (addr == NULL) ||
        (!hostname) || (!hostname[0]) ||
        (strlen(hostname) >= DNS_MAX_NAME_LENGTH))
    {
        return ERR_VAL;
    }

#if LWIP_HAVE_LOOPIF
    if (strcmp(hostname, "localhost") == 0)
    {
        addr->addr = htonl(INADDR_LOOPBACK);
        return ERR_OK;
    }
#endif /* LWIP_HAVE_LOOPIF */

    /* host name already in octet notation? set ip addr and return ERR_OK
     * already have this address cached? */
    if (((addr->addr = inet_addr(hostname)) != INADDR_NONE) ||
        ((addr->addr = dns_lookup(hostname)) != INADDR_NONE))
    {
        return ERR_OK;
    }

    /* queue query with specified callback */
    return dns_enqueue(hostname, found, callback_arg);
}

#endif /* LWIP_DNS */
